This invention relates generally to nuclear reactors and more particularly, to apparatus for repairing jet pump assemblies within a nuclear reactor pressure vessel.
A reactor pressure vessel (RPV) of a boiling water reactor (BWR) typically has a generally cylindrical shape and is closed at both ends, e.g., by a bottom head and a removable top head. A top guide typically is spaced above a core plate within the RPV. A core shroud, or shroud, typically surrounds the core and is supported by a shroud support structure. Particularly, the shroud has a generally cylindrical shape and surrounds both the core plate and the top guide. There is a space or annulus located between the cylindrical reactor pressure vessel and the cylindrically shaped shroud.
In a BWR, hollow tubular jet pumps positioned within the shroud annulus provide the required reactor core water flow. The upper portion of the jet pump, known as the inlet mixer, is laterally positioned and supported against two opposing rigid contacts within restrainer brackets by a gravity actuated wedge. The restrainer brackets support the inlet mixer by attaching to the adjacent jet pump riser pipe. The purpose of the gravity actuated wedge is to maintain contact between the inlet mixer and the restrainer bracket. The wedge works in cooperation with two set screws which are tack welded to the restrainer bracket to maintain contact with the inlet mixer. The flow of water through the jet pumps typically includes pressure fluctuations that are caused by various sources in the reactor system. The pressure fluctuations may have frequencies close to one or more natural vibration modes of the jet pump piping. The jet pump piping stability depends on the tight fit-up, or contact, of the restrainer brackets and the inlet mixers. Operating thermal gradients, hydraulic loads, and fluctuations in the hydraulic loads can overcome the lateral support provided by the gravity wedge, allowing gaps or clearances to develop at the opposing two fixed contacts or set screws. Particularly, the tack welds can break and the set screws can loosen permitting the jet pump to vibrate within the restrainer bracket. The loss of contact between the inlet mixer and the restrainer bracket can change the jet pump natural frequency to match some excitation frequency in the system, causing vibration of the piping and exerting increased loads which may cause cyclic fatigue cracking and wear of the piping supports, which can result in degradation from wear and fatigue at additional jet pump structural supports.
To overcome this problem, gravity wedge supports have been used at locations where clearances have developed in restrainer bracket contacts. The gravity wedge support employed a sliding wedge and a fixed bracket mount that engaged the jet pump restrainer bracket. These gravity wedges were only applicable to restrainer bracket/inlet mixer gap widths from about 1.0 to 2.0 inches, as space was required for a wedge with sufficient weight to give the desired support load. Another solution which was implemented was to reinforce the welded attachment of the two set screws to the restrainer bracket, then reset the inlet mixer against the set screws when the jet pump is reassembled. However, this procedure causes significant downtime and also requires disassembling the jet pumps.
It would be desirable to provide an apparatus for restoring the tight rigid fit-up provided between the inlet mixer and the adjacent restrainer bracket, replacing the support function of the existing screw type contacts. It would also be desirable to provide an apparatus that can be installed in restrainer bracket/inlet mixer gap widths as small as xc2xc inch and provide a continuous adjustment for possible alignment variations between the mixer and diffuser. Further, it would be desirable to provide an apparatus that compensates for after-installation changes in the interface between the mixer and diffuser. Additionally, it would be desirable to provide an apparatus that can be remotely installed by attachment to the existing restrainer bracket without disassembling the inlet mixer.
In one embodiment of the present invention, a remotely installable piping support device couples to the restrainer bracket of a jet pump inlet mixer at a position adjacent an existing screw type contact, typically, a set screw. The remotely installable piping support device includes a left lever arm and a right lever arm. The right lever arm is coupled to the left lever arm. The device also includes a clamp spring which forces the end portions of the lever arms together. The left and right lever arms are movable in relation to one another about the clamp spring as the clamp spring is torsionally deflected. At least one tapered first wedge segment is mounted on the first lever arm, and at least one tapered second wedge segment is mounted on the second lever arm. The tapered second wedge segment slideably engages the tapered first wedge segment to fill any clearances that develop between an inlet mixer and a restrainer bracket. In an exemplary embodiment, the clamp spring is a spiral wound spring integral with one of the lever arms, and the remotely installable piping support device is referred to as a spring wedge.
During installation, the spring wedge is spread apart to an open position utilizing an installation tool. The open spring wedge is positioned around a set screw and the plier tool is removed allowing the spring clamp to cause the first lever arm and second lever arm to close together. The tapered first wedge segment and the tapered second wedge segment slideably engage to fill a gap between the mixer inlet and the restrainer bracket, thus, providing a tight fit-up between the mixer inlet and the restrainer bracket.
The above described spring wedge restores the tight rigid fit-up between the inlet mixer and the adjacent restrainer bracket, enhancing the support function of existing screw type contacts. Additionally the spring wedge is remotely installed by insertion between an existing restrainer bracket and an existing installed inlet mixer. Furthermore, the spring wedge is configured fit in jet pumps which have a restrainer bracket/inlet mixer gap width as small as xc2xc inch.